Re: [PATCH] uart:8250: change lock order in serial8250_do_startup()

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Fri, Aug 14, 2020 at 10:38:02AM +0900, Sergey Senozhatsky wrote:
> We have a number of "uart.port->desc.lock vs desc.lock->uart.port"
> lockdep reports coming from 8250 driver; this causes a bit of trouble
> to people, so let's fix it.
> 
> The problem is reverse lock order in two different call paths:
> 
> chain #1:
> 
>  serial8250_do_startup()
>   spin_lock_irqsave(&port->lock);
>    disable_irq_nosync(port->irq);
>     raw_spin_lock_irqsave(&desc->lock)
> 
> chain #2:
> 
>   __report_bad_irq()
>    raw_spin_lock_irqsave(&desc->lock)
>     for_each_action_of_desc()
>      printk()
>       spin_lock_irqsave(&port->lock);
> 
> Fix this by changing the order of locks in serial8250_do_startup():
>  do disable_irq_nosync() first, which grabs desc->lock, and grab
>  uart->port after that, so that chain #1 and chain #2 have same lock
>  order.
> 
> Full lockdep splat:
> 
>  ======================================================
>  WARNING: possible circular locking dependency detected
>  5.4.39 #55 Not tainted
>  ------------------------------------------------------
>  swapper/0/0 is trying to acquire lock:
>  ffffffffab65b6c0 (console_owner){-...}, at: console_lock_spinning_enable+0x31/0x57
> 
>  but task is already holding lock:
>  ffff88810a8e34c0 (&irq_desc_lock_class){-.-.}, at: __report_bad_irq+0x5b/0xba
> 
>  which lock already depends on the new lock.
> 
>  the existing dependency chain (in reverse order) is:
> 
>  -> #2 (&irq_desc_lock_class){-.-.}:
>         _raw_spin_lock_irqsave+0x61/0x8d
>         __irq_get_desc_lock+0x65/0x89
>         __disable_irq_nosync+0x3b/0x93
>         serial8250_do_startup+0x451/0x75c
>         uart_startup+0x1b4/0x2ff
>         uart_port_activate+0x73/0xa0
>         tty_port_open+0xae/0x10a
>         uart_open+0x1b/0x26
>         tty_open+0x24d/0x3a0
>         chrdev_open+0xd5/0x1cc
>         do_dentry_open+0x299/0x3c8
>         path_openat+0x434/0x1100
>         do_filp_open+0x9b/0x10a
>         do_sys_open+0x15f/0x3d7
>         kernel_init_freeable+0x157/0x1dd
>         kernel_init+0xe/0x105
>         ret_from_fork+0x27/0x50
> 
>  -> #1 (&port_lock_key){-.-.}:
>         _raw_spin_lock_irqsave+0x61/0x8d
>         serial8250_console_write+0xa7/0x2a0
>         console_unlock+0x3b7/0x528
>         vprintk_emit+0x111/0x17f
>         printk+0x59/0x73
>         register_console+0x336/0x3a4
>         uart_add_one_port+0x51b/0x5be
>         serial8250_register_8250_port+0x454/0x55e
>         dw8250_probe+0x4dc/0x5b9
>         platform_drv_probe+0x67/0x8b
>         really_probe+0x14a/0x422
>         driver_probe_device+0x66/0x130
>         device_driver_attach+0x42/0x5b
>         __driver_attach+0xca/0x139
>         bus_for_each_dev+0x97/0xc9
>         bus_add_driver+0x12b/0x228
>         driver_register+0x64/0xed
>         do_one_initcall+0x20c/0x4a6
>         do_initcall_level+0xb5/0xc5
>         do_basic_setup+0x4c/0x58
>         kernel_init_freeable+0x13f/0x1dd
>         kernel_init+0xe/0x105
>         ret_from_fork+0x27/0x50
> 
>  -> #0 (console_owner){-...}:
>         __lock_acquire+0x118d/0x2714
>         lock_acquire+0x203/0x258
>         console_lock_spinning_enable+0x51/0x57
>         console_unlock+0x25d/0x528
>         vprintk_emit+0x111/0x17f
>         printk+0x59/0x73
>         __report_bad_irq+0xa3/0xba
>         note_interrupt+0x19a/0x1d6
>         handle_irq_event_percpu+0x57/0x79
>         handle_irq_event+0x36/0x55
>         handle_fasteoi_irq+0xc2/0x18a
>         do_IRQ+0xb3/0x157
>         ret_from_intr+0x0/0x1d
>         cpuidle_enter_state+0x12f/0x1fd
>         cpuidle_enter+0x2e/0x3d
>         do_idle+0x1ce/0x2ce
>         cpu_startup_entry+0x1d/0x1f
>         start_kernel+0x406/0x46a
>         secondary_startup_64+0xa4/0xb0
> 
>  other info that might help us debug this:
> 
>  Chain exists of:
>    console_owner --> &port_lock_key --> &irq_desc_lock_class
> 
>   Possible unsafe locking scenario:
> 
>         CPU0                    CPU1
>         ----                    ----
>    lock(&irq_desc_lock_class);
>                                 lock(&port_lock_key);
>                                 lock(&irq_desc_lock_class);
>    lock(console_owner);
> 
>   *** DEADLOCK ***
> 
>  2 locks held by swapper/0/0:
>   #0: ffff88810a8e34c0 (&irq_desc_lock_class){-.-.}, at: __report_bad_irq+0x5b/0xba
>   #1: ffffffffab65b5c0 (console_lock){+.+.}, at: console_trylock_spinning+0x20/0x181
> 
>  stack backtrace:
>  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.4.39 #55
>  Hardware name: XXXXXX
>  Call Trace:
>   <IRQ>
>   dump_stack+0xbf/0x133
>   ? print_circular_bug+0xd6/0xe9
>   check_noncircular+0x1b9/0x1c3
>   __lock_acquire+0x118d/0x2714
>   lock_acquire+0x203/0x258
>   ? console_lock_spinning_enable+0x31/0x57
>   console_lock_spinning_enable+0x51/0x57
>   ? console_lock_spinning_enable+0x31/0x57
>   console_unlock+0x25d/0x528
>   ? console_trylock+0x18/0x4e
>   vprintk_emit+0x111/0x17f
>   ? lock_acquire+0x203/0x258
>   printk+0x59/0x73
>   __report_bad_irq+0xa3/0xba
>   note_interrupt+0x19a/0x1d6
>   handle_irq_event_percpu+0x57/0x79
>   handle_irq_event+0x36/0x55
>   handle_fasteoi_irq+0xc2/0x18a
>   do_IRQ+0xb3/0x157
>   common_interrupt+0xf/0xf
>   </IRQ>

I guess we may add some tags here

Fixes: 768aec0b5bcc ("serial: 8250: fix shared interrupts issues with SMP and RT kernels")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Raul Rangel <rrangel@google.com>
BugLink: https://bugs.chromium.org/p/chromium/issues/detail?id=1114800
Link: https://lore.kernel.org/lkml/CAHQZ30BnfX+gxjPm1DUd5psOTqbyDh4EJE=2=VAMW_VDafctkA@mail.gmail.com/T/#u

Since above below a nit-pick after addressing these,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Thanks!

> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  drivers/tty/serial/8250/8250_port.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 09475695effd..67f1a4f31093 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2275,6 +2275,11 @@ int serial8250_do_startup(struct uart_port *port)
>  
>  	if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
>  		unsigned char iir1;

> +		bool irq_shared = up->port.irqflags & IRQF_SHARED;

I'm wondering why we need a temporary variable? This flag is not supposed to be
changed in between, can we leave original conditionals?

Nevertheless I noticed an inconsistency of the dereference of the flags which
seems to be brough by dfe42443ea1d ("serial: reduce number of indirections in
8250 code").

I think we can stick with newer:

		if (port->irqflags & IRQF_SHARED)

> +
> +		if (irq_shared)
> +			disable_irq_nosync(port->irq);
> +
>  		/*
>  		 * Test for UARTs that do not reassert THRE when the
>  		 * transmitter is idle and the interrupt has already
> @@ -2284,8 +2289,6 @@ int serial8250_do_startup(struct uart_port *port)
>  		 * allow register changes to become visible.
>  		 */
>  		spin_lock_irqsave(&port->lock, flags);
> -		if (up->port.irqflags & IRQF_SHARED)
> -			disable_irq_nosync(port->irq);
>  
>  		wait_for_xmitr(up, UART_LSR_THRE);
>  		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
> @@ -2297,9 +2300,9 @@ int serial8250_do_startup(struct uart_port *port)
>  		iir = serial_port_in(port, UART_IIR);
>  		serial_port_out(port, UART_IER, 0);
>  
> -		if (port->irqflags & IRQF_SHARED)
> -			enable_irq(port->irq);
>  		spin_unlock_irqrestore(&port->lock, flags);
> +		if (irq_shared)
> +			enable_irq(port->irq);
>  
>  		/*
>  		 * If the interrupt is not reasserted, or we otherwise
> -- 
> 2.28.0
> 

-- 
With Best Regards,
Andy Shevchenko